Currently, Li-ion based batteries are believed to be the most promising battery system for HEV/PHEV/EV applications due to their high energy density, lack of hysteresis and low self-discharge currents. However, a formidable challenge lies in fast charging of Li-ion batteries where standard charging techniques such as CC-CV, if used for fast charging, can result in damage to the battery due to the large currents passed through the battery. These large currents result in overpotentials and mechanical stress in the battery that can accelerate the aging process of the battery and resulting in reduced lifetime.
There is an increasing trend towards the electrification of the automobile, and most car manufacturers have announced plans to produce plug-in hybrid and electric vehicles. Besides other technological challenges, one important aspect of an electric vehicle is the time needed to recharge the battery pack. Advanced battery management systems need to provide adequate charging strategies for refueling the battery pack in a fast and reliable manner.
Although it is widely recognized that an appropriate charging strategy of the battery is critical for preventing damage and performance degradation, in general only current and voltage limits are considered during the charging process. For example, voltage limits may be too conservative for new batteries and possibly inappropriate for use with aged batteries due to the changed behavior.
Most charging strategies are ad hoc methods, where certain design parameters determine the major part of some rule based control design. Fast charging of batteries is a popular research topic in the field of electrochemistry, however, this problem has received very little attention in the field of control systems. Popular charging strategies are constant-current/constant-voltage (CC/CV), pulse current charging, and pulse voltage charging, with CC/CV being the most wide-spread method to recharge Li-ion batteries. However, the existing approaches fail to achieve a maximum performance as determined by the electrochemistry of the battery.
In an ideal battery, and without limitation of the charging unit, one could pass all the charge needed to bring a battery from one state of charge (SOC) to another SOC instantaneously. Kinetic limitations in real batteries, however, allow only a finite current to be passed through a battery. Many internal processes of the battery have an influence on the charge transfer capabilities, e.g. finite diffusion rate of lithium ions in the electrolyte, reduction/oxidation of materials other than the active material, formation of resistive films on the active particle surface, and charge transfer limitation between the electrolyte and the active material. The faster the charge transfer is forced to happen, the more strongly these processes affect the health of the battery. Cell manufacturers thus always provide additional information about utilization constraints on their cells. These constraints mostly involve limits on the maximum charge or discharge current, limits of lower and upper cut-off voltages, and the operating temperature domain. Some manufacturers provide these limits at different operating ambient temperatures. All these limits are geared towards the CC/CV charging method, and hence are rather conservative, since the limits are specified for the complete lifetime of the battery.
In light of the foregoing limitations, improved methods for charging batteries, and lithium-ion batteries in particular, that reduce the time required to charge the battery while avoiding excessive degradation and aging of the battery during the charging process would be beneficial. Additionally, systems for controlling the charging process for batteries in an improved manner would be beneficial.